Got the 100-400 today so I did a quick test (I say quick, couple of hours ha) against the 70-200 GM (same tests I did with the F4 vs 2.8 70-200s and the 1.4x TC)
From 100mm to 180mm using the same aperture, iso & shutter on the two I cant tell them apart, The 70-200 goes slightly soft at 200mm which gives the 100-400 the edge. Wide open at 100mm (2.8 vs 4.5) and 135mm (2.8 vs 5) the 100-400 just has the edge as I expected, the 70-200 goes slightly soft but its only in the corners so could be my dodgy set up
Now to waste a few more hours throwing the sigma 150-600 in to the mix